Cardiac function varies during respiration, a phenomenon referred to as the “respiration effect.” Pressures in the right atrium and thoracic vena cava depend on intrapleural pressure (Ppl). During inspiration, the chest wall expands and the diaphragm lowers. This causes a fall in Ppl that leads to expansion of the lungs and cardiac chambers (e.g., right atrium and right ventricle), and a reduction in right atrial and ventricular pressures. As right atrial pressure falls during inspiration, the pressure gradient for venous return to the right ventricle increases. During expiration, the opposite occurs. Thus, the net effect of respiration is that increasing the rate and depth of ventilation facilitates venous return and ventricular stroke volume.
The respiration effect is typically seen earlier in the right ventricle than in the left ventricle, since inspiration and expiration tend to affect the hemodynamics of the right ventricle more directly than that of the left ventricle. The effect of respiration on the right side of the heart is subsequently observed on the left side of the heart, typically after a delay of about one cardiac cycle or more, as changes in the mechanical function of the right side of the heart are observed on the left side of the heart in the next few cardiac cycles. This time lag may become more pronounced in certain patients as cardiopulmonary functions deteriorate (for example without limitation, due to the progression of heart failure, pulmonary edema, and pulmonary hypertension).
To date, various methods have been proposed for detecting respiration with an implantable medical device (IMD). For example, minute ventilation sensors have been used to measure respiration by monitoring cyclic changes in transthoracic impedance that occur during respiration. Intracardiac electrogram (EGM) amplitude modulation has also been used to monitor respiration. A technique for monitoring respiration that uses blood pressure signals has also been proposed.
The respiration effect may cause fluctuations in a number of hemodynamic parameters that may be the subject of monitoring and/or the basis for therapy decisions. Such fluctuations may affect the ability of a physician (or an IMD) to interpret the monitored hemodynamic parameters and/or to provide (or deliver) appropriate therapy.